Central Europe in the times between the decline of the Western Roman Empire and the rise of the Christian Frankish rule is something that interests me a lot, though I don't know of many contemporary literary sources from the region. if you know of any interesting pieces, I'd be very appreciative, though I know not a whole ton was written down.
Assuming you mean things that were written by and not about germanic people, there is very little. No germanic language existed in written form before the contact with Rome and Christianity.
The Gothic or Wulfila Bible is the most ancient piece of literature written in a germanic language. There are some runestones that predate the carolingian times, but I don't know if that can be considered literature. The Lex Salica contains fragments written in frankish, but is predominantly latin.
Regarding literature about pre-carolinigian germanic people, the histories written by Gregory of Tours and Isidor of Sevilla come to mind, but they are written in latin. As with all primary sources, they are not to be taken at face value. The chronic of Fredegar could also be mentioned, but is in latin as well, as is the Liber Historiae Francorum.